The present invention relates to improved low-sulfur fuel compositions which exhibit reduced static and good lubricity.
As a consequence of the refinery processes employed to reduce diesel sulfur and aromatics content, the majority of ultra-low sulfur Diesel (50 ppm w/w sulfur max.), fuels marketed today require treatment with an additive to restore lubricity. A further consequence of the removal of sulfur from the fuel is a reduction in its electrical conductivity. The more highly insulating nature of the ultra-low sulfur fuel presents additional risk to refiners, oil companies and consumers alike, due to the potential build-up of high static charges. Static charging can occur during pumping operations. In such operations, the flow of low conductivity liquid through pipes and filters, combined with the disintegration of a liquid column and splashing during high speed tank loading can result in static charging. Such static charging can result in spark electrical discharge, with catastrophic potential in highly flammable environments.
The dissipation of static charge by, for instance, the addition of a conductivity-enhancing additive to the fuel is becoming an increasingly important issue as diesel sulfur levels are progressively reduced. Field experience has shown that the conductivity additive should provide enhanced conductivity within the fuel to avoid the build-up of static charge and should have no undesirable side effects. Such side effects could include: degrading the properties of the base fuel; interacting with crankcase lubricating oils; and reducing the performance benefit of lubricity or other fuel additives.
The global legislative drive to reduce automotive emissions has resulted in a widespread reduction in diesel fuel sulfur levels. The sulfur present in diesel fuels has been demonstrated to have several adverse environmental consequences.
In order to meet emissions and fuel efficiency goals, automotive original equipment manufacturers are investigating the use of NOx traps, particulate traps and direct injection technologies. Such trap and catalyst systems tend to be intolerant to sulfur, thus a further category for diesel fuel has been introduced for markets with further advanced requirements for emission control. This new category of diesel is used to define the cleanest burning fuels required to enable sophisticated after-treatment technologies to be used. The new xe2x80x9cCategory 4 diesel fuelxe2x80x9d specifies xe2x80x9csulfur-freexe2x80x9d diesel (5-10 ppm maximum) (Reference World-Wide Fuel Charter, April 2000, Issued by ACEA, Alliance of Automobile Manufacturers, EMA and JAMA). This is the required specification to ensure compliance with emissions requirements over the full useful life of this latest technological generation of vehicles. Low sulfur and ultra-low sulfur fuels are also becoming increasingly necessary for conventional diesel engines, as governments introduce further legislation for the reduction in particulate matter.
A primary consequence of the removal of sulfur from fuels is the removal of much of the natural lubricating properties of the fuel. As a secondary consequence, the levels of conducting species such as aromatics and heteroatoms is reduced during hydrodesulfurization processing at the refinery. Generally, as the concentration of conducting species in a fuel increases, so do both the electrical conductivity and the static charging potential. This continues until a maximum potential for charging is reached. As conductivity continues to rise beyond this critical value, the charging effect is ameliorated by dissipation through the fuel and the spark discharge tendency is reduced. Static charging is most significant therefore in fuels with conductivity around or below the aforementioned critical value.
The potentially catastrophic problems associated with static charging in fuel pumping applications were first addressed in the jet-fuel industry, in which the necessary high pumping rates of the low sulfur (xcx9c400 ppm w/w UK average) resulted in inevitable static charging. The static-charging problem is such that the standard specifications for jet fuels include the addition of an additive to increase their conductivity. The treatment is usually with an industry standard specified additive at the rate of xcx9c2 ppm w/w. The minimum conductivity requirement for jet kerosene is generally quoted as 50 picosiemens mxe2x88x921.
In diesel applications, the presence of high concentrations of sulfur containing molecules ( greater than 500 ppm) has been sufficient to give significant intrinsic conductivity, such that static charging problems have not been a problem. However, as sulfur levels in diesel are reduced, the risk of static charging during pumping operations has increased significantly. This has resulted in several reports of road tanker explosions in Europe following the introduction of Ultra-Low Sulfur Diesel (ULSD), despite the use of grounding leads. These incidents were specifically attributed to static charge induced spark ignition of fuel vapor, during fuel transfer operations.
Increasingly, the addition of lubricity additives to diesel is effected at the refinery. It is therefore desirable to address the lubricity and conductivity problems associated with ULSD with a single additive package.
The oil and additive industries have developed a wide range of tests to evaluate the no-harm performance of additive packages and components. The present invention provides a new lubricity 1 anti-static additive package suitable for meeting the requirements of ULSD Fuel.
The additive packages of the present invention provide protection against the build-up of static charge without significant undesirable effects on the fuel or lubricity additive performance. The possibility of interaction between anti-static additives and typical lubricity packages, and the resulting potential influence over product performance and ultimately fuel quality, is minimized.
Various lubricity additives are known in the art. U.S. Pat. No. 5,833,722, Davies et al., Nov. 10, 1998, discloses enhancing the lubricity of low sulphur fuels by incorporation of a lubricity enhancing additive, such as a carboxylic acid ester, in combination with a nitrogen compound carrying one or more substituents of the formula  greater than NR13 where R13 represents a hydrocarbyl group containing 8 to 40 carbon atoms. European Patent Application 798 364, Oct. 1, 1997, discloses a diesel fuel additive comprising a salt of a carboxylic acid and an aliphatic amine, or an amide obtained by dehydration-condensation thereof. The additive reduces the amount of deposits and improves lubricity of the fuel. It is also said to impart anti-wear property to diesel fuel of low sulfur content.
The present invention provides a fuel composition exhibiting improved anti-static properties comprising:
a liquid fuel which contains less than 500 parts per million by weight sulfur;
0.001 to 1 part per million by weight of at least one hydrocarbyl monoamine or N-hydrocarbyl-substituted poly(alkyleneamine); and
20 to 500 parts per million by weight of at least one fatty acid containing 8 to 24 carbon atoms or an ester thereof with an alcohol or polyol of up to 8 carbon atoms.